Toll-Like Receptors and Aseptic Loosening of Hip Endoprosthesis—A Potential to Respond against Danger Signals? Tuomas La ¨hdeoja, 1 Jukka Pajarinen, 1,2,3 Vesa-Petteri Kouri, 1 Tarvo Sillat, 1 Jari Salo, 2 Yrjo ¨ T. Konttinen 1,4,5 1 Institute of Clinical Medicine, Department of Medicine, Biomedicum Helsinki, Helsinki University Central Hospital, P.O. Box 700 (Haartmaninkatu 8), Helsinki FIN-00029 HUS, Finland, 2 Department of Orthopaedics and Traumatology, Helsinki University Central Hospital, Helsinki, Finland, 3 Department of Anatomy, University of Helsinki, Helsinki, Finland, 4 ORTON Orthopaedic Hospital of the ORTON Foundation, Helsinki, Finland, 5 COXA Hospital for the Joint Replacement, Tampere, Finland Received 2 February 2009; accepted 9 July 2009 Published online 1 September 2009 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/jor.20979 ABSTRACT: Bacterial remnants and subclinical biofilms residing on prosthesis surfaces have been speculated to play a role in hip implant loosening by opsonizing otherwise relatively inert wear particles. The innate immune system recognizes these microbial pathogen-associated molecular patterns (PAMPs) using Toll-like receptors (TLRs). Our objective was to evaluate the possible presence of TLRs in aseptic synovial membrane-like interface tissue. Bacterial culture-negative, aseptic (n ¼ 4) periprosthetic synovial membrane-like tissue was compared to osteoarthritis synovial membrane (n ¼ 5) for the presence of cells positive for all known human functional TLRs, stained using specific antibodies by immunohistochemistry, and evaluated using morphometry. In comparison to osteoarthtritic synovium, the number of TLR- positive cells was found to be increased in the aseptic setting, reflecting the considerable macrophage infiltration to the tissues investigated. Thus aseptic periprosthetic tissue seems to be very reactive to PAMPs. It has been recently recognized that TLR do not only respond to traditional PAMPs, but also to endogenous alarmings or danger signals released from necrotic and activated cells. Alarming-TLR interaction in the periprosthetic tissue might be a novel mechanism of aseptic loosening of endoprosthesis. ß 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 28:184 – 190, 2010 Keywords: hip prosthesis; prosthesis failure; foreign-body reaction; Toll-like receptors; lipopolysaccharide Replacement arthroplasties are used to prevent crippling and to preserve working capacity, usually with great success. 1 Despite achievements in material technology and surgical technique, an aseptic loosening of prosthesis components remains one of the major long-term complications of large joint replacement surgery. 2,3 As increasing numbers of young and active patients are operated, the prosthesis revision rates due to aseptic loosening are also expected to increase. 4,5 It is widely accepted that aseptic osteolysis is primary driven by wear particles typically generated at titanium alloy stem/polyethylene cup or bone/bone cement inter- faces. 2,6 In the peri-implant tissues, wear particles of various natures are recognized and phagocytosed by monocyte/macrophages which are then induced to produce several pro-inflammatory cytokines. These cytokines lead to increased osteoclast differentiation, giant cell formation, and ultimately to osteolysis. 7,8 The molecular mechanisms by which these wear particles are recognized by monocyte/macrophages are, however, poorly understood. It has recently been suggested that it is not the wear particles per se that cause macrophage activation. The particles only serve as high-surface area, foreign body platforms to which bacterial structural components— such as lipopolysaccharide (LPS) or biofilms composed of living bacteria—adhere and that, in fact, it is these bacterial components that are recognized by monocyte/ macrophages and that cause them to produce pro- inflammatory cytokines while the particles, as such, are relatively inert. 9–11 These types of bacterial structural components, named pathogen-associated molecular patterns (PAMPs), are typically recognized by pattern recognition receptors of the innate immunity, most importantly by so-called Toll-like receptors (TLRs). 12–14 TLRs are a family of nine different germ-line encoded transmembrane receptor proteins that enable the innate immune system to recognize numerous evolutionary well-conserved bacte- rial-, viral-, and fungal-derived structure (Table 1). Typically, TLR stimulation with an appropriate ligand causes cell activation and production of several pro- inflammatory cytokines, which then further activates the innate and also adaptive immunity. Interestingly, these same cytokines have been linked to the patho- genesis of aseptic loosening. 15,16 It is hypothesized that bacterial product-coated wear particles are in the peri-implant tissues initially recognized by different TLRs. To our knowledge, only the presence and upregulation of two TLR proteins in the synovial membrane-like interface tissue has thus far been demonstrated. 17 Thus, the aim of this study was to investigate the eventual presence of all known human functional TLR receptors in the periprosthetic synovial membrane-like tissues, compared to mildly inflamed osteoarthritic synovial membrane. MATERIALS AND METHODS Patients and Samples The project was accepted by the Ethical Committee of the Helsinki and Uusimaa Hospital District. Tissue samples of synovial membrane-like interface tissue formed around loosening total hip replacement implants were obtained from hip revision arthroplasty operations performed due to bacterial and fungal culture-negative aseptic loosening of the prosthesis. Control synovial membrane samples were obtained from primary arthroplasty operations from patients with 184 JOURNAL OF ORTHOPAEDIC RESEARCH FEBRUARY 2010 Correspondence to: Yrjo ¨ T. Konttinen (T: þ358-9-191 25210; F: þ358-9-191 25218; E-mail: yrjo.konttinen@helsinki.fi) ß 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc.